Today's state-of-the-art visible-spectrum light-emitting diodes (LEDs) and laser diodes (LDs) in the ultraviolet to green (380 nm to 550 nm) regime are based on InGaN active layers grown pseudomorphic to wurtzite GaN. This is true whether the growth substrate is GaN itself, or a foreign substrate such as sapphire or SiC, since in the latter cases GaN-based template layers are employed. Because the lattice constants of GaN and InN are significantly different, InGaN grown pseudomorphically on GaN substrates or layers has significant stress, where the magnitude increases as the In/Ga ratio in the InGaN layer increases.
The built-in stress within the InGaN active layers can make it difficult to achieve high quality material and good device operation. Obtaining high quality material and good device operation becomes progressively more difficult as the InN mole fraction increases, which is a requirement for longer wavelength devices. In addition, for c-plane grown devices, increasing the InN mole fraction also increases the built-in electric fields across the active layers due to spontaneous and piezoelectric polarization fields, reducing the overlap between electrons and holes and decreasing the radiative efficiency. Moreover, there is evidence that material breakdown occurs once the stress level becomes too high, resulting in so-called “phase separation” (see N. A. El-Masry, E. L. Piner, S. X. Liu, and S. M. Bedair, “Phase separation in InGaN grown by metalorganic chemical vapor deposition,” Appl. Phys. Lett., vol. 72, pp. 40-42, 1998). Phase separation is exhibited beyond a critical limit of a certain InN mole fraction combined with a certain layer thickness. Such a limit is commonly observed for InGaN layers of about 10% InN grown more than 0.2 μm thick, for example, resulting in “black” or “grey” wafers.
The use of substrates comprising non-polar (1-100), (11-20), and semi-polar planes of GaN can address some of the problems above. In particular, for certain growth planes, the combined spontaneous and piezoelectric polarization vector can be reduced to zero or near-zero, eliminating the electron-hole overlap problem prevalent in c-plane-based devices. Also, improved material quality with higher InN mole fraction can be observed, such as is demonstrated in semi-polar material, which has resulted in continuous-wave (cw) true-green laser diodes (LDs) (see Enya et al., “531 nm green lasing of InGaN based laser diodes on semi-polar {20-21} free-standing GaN substrates,” Appl. Phys. Express 2, 082101, 2009; J. W. Raring et al., “High-efficiency blue and true-green-emitting laser diodes based on non-c-plane oriented GaN substrates,” Appl. Phys. Express 3, 112101 (2010)). However, the performance of longer-wavelength devices grown on these structures still suffers considerably compared to that of their shorter-wavelength counterparts. Also, it is not clear that growth plane orientation would eliminate the material quality problems associated with strain. Indeed, recent characterization of semi-polar (Al,In,Ga)N heterostructures reveals the formation of a large density of misfit dislocations at heterointerfaces between AlGaN and GaN, for example (see A. Tyagi et al., “Partial strain relaxation via misfit dislocation generation at heterointerfaces in (Al,In)GaN epitaxial layers grown on semipolar (11-22) GaN free standing substrates,” Appl. Phys. Lett. 95, 251905, 2009). These dislocations are likely to act as non-radiative recombination centers, and these dislocations may also provide potential degradation mechanisms which may prevent long-life operation (e.g., as is necessary for applications such as solid-state lighting). Further, reported external quantum efficiencies vs. wavelength for LEDs generally show a strong reduction in external quantum efficiency with increasing InN mole fraction, which is often referred to as the “green gap,” regardless of growth plane orientation.